Cellulose pulp is normally washed after separation of the pulping liquor at the conclusion of the digestion, before it is passed on to subsequent chemical treatment stages, such as bleaching. The pulping liquor contains substantial quantities of dissolved impurities, which react with treating chemicals, and if these impurities are not removed, or the concentration thereof at least greatly reduced, subsequent chemical treatments applied to the pulp, particularly bleaching, may be relatively inneffective, because of the consumption of such chemicals by the impurities. The impurities therefore not only reduce the bleaching effect, but may also require the addition of larger amounts of the treating agents, which are largely wasted. Dissolved impurities present in the pulping liquor after digestion include the pulping chemicals and the organic substances formed in the course of the pulping process which are water-soluble and become dissolved in the liquor. The dissolved impurities which accompany the cellulose pulp suspension must be removed by the washing.
Accordingly, the cellulose pulp washing system is designed to remove the dissolved impurities, and this is normally done by replacing the aqueous suspending liquor containing dissolved impurities with a fresh or relatively pure aqueous suspending liquid, substantially free from such impurities, or at least having a lower content thereof than the aqueous suspension from the pulper or digester.
Cellulose pulp slurries are normally washed in multiple washing stages. Usually, three or four washing stages are used. When a multiplicity of washing stages are employed, the stages are arranged in series and counter-current pulp washing techniques are used to increase efficiency. The fresh washing liquid is supplied to the last stage, and then progresses forwardly towards the first washing stage, in series along the line of washing stages. In this way, the washing liquor containing a progressively greater proportion of dissolved impurities is utilized to wash the cellulose pulp fiber slurry containing a progressively greater proportion of impurities, so that the washing liquor is re-used efficiently from stage to stage. In the final washing stage, the washing liquid, often pure water, can be expected to remove substantially all of the remaining dissolved impurities. The dissolved impurities which remain in the discharged pulp slurry are referred to as washing loss or soda loss. The spent washing liquor containing the impurities dissolved from the starting cellulose pulp suspension is then collected, and the solids content can be recovered as desired.
For optimum washing efficiency, it is obviously desirable to carry out the washing with the least possible amount of washing loss, and the least possible dilution of the recovered black liquor. The key point is that a balance must be maintained between the amount of washing water used and desired pulp cleanliness. High washing losses are of course disadvantageous for many reasons. The chemicals that lost are economically important, and their loss increases the cost of operation. Hence, it is desirable that the washing losses be kept as low as possible, while using a minimum of washing water. High washing losses also lead to problems in the subsequent treatment of the pulp. If an excessively large amount of dissolved impurities accompanies the pulp from the washing stage to the screening stage, and then on to the bleaching stage, there may be an unduly high consumption of bleaching chemicals and other treating chemicals. In terms of energy costs, overuse of shower water is expensive because all excess shower water used in the washing process must eventually be evaporated in the recovery evaporators.
Variations in washing losses can be caused by a number of different factors. For example, the volume of pulp slurry coming into the washing system can increase due to an increased production rate in the digester. This would increase the amount of impurities which must be removed by washing and can also increase the pulp consistency in the washers. Even at a stable pulp production rate, variations in the processing conditions in the digester, affected by factors such as the composition of the black liquor, the degree of delignification of the pulp cellulose and the make-up of the charge to the digester, can cause changes in the amount of impurities in the pulp slurry.
Cellulose pulp washing systems are highly specialized, and a special terminology has been developed to refer to various aspects thereof. Several of the more important and more commonly encountered terms are defined below.
Original black liquor: The pulping liquor which serves as a suspending medium for the cellulose pulp in the digester, at the conclusion of the pulping process. This liquor contains dissolved pulping chemicals, and also inorganic and organic materials produced as by-products from the pulping reaction, including organic water-soluble material dissolved from the wood.
Recovered black liquor or release liquor: The black liquor which is obtained subsequent to washing the pulp, containing the dissolved solids recovered by washing from the original black liquor. The recovered black liquor is passed to the evaporation stage, where the liquor is concentrated to a heavy black liquor or thick black liquor.
Total Soda Loss: The quantity of original black liquor sodium salts leaving the final wash stage with the pulp suspension. Soda Loss is often expressed in terms of pounds of salt cake (NA.sub.2 SO.sub.4) per ton of pulp.
Washable Soda Loss (SL): That portion of the total soda loss which can be recovered by displacement washing.
Washing loss: The quantity of original black liquor dissolved solids which remains with the washed cellulose pulp suspension, after the washing has been completed. The washing loss varies according to the pulping process and the analytical technique used to determine it.
Dilution factor (DF): The difference between the volume of recovered black liquor and original black liquor, i.e., the quantity of black liquor after the addition of wash water in excess of the quantity of original black liquor charged. Dilution factor is often expressed in terms of tons or cubic meters of liquid per ton of pulp.
Displacement Ratio (DR): The fraction of the liquor entering with the pulp which is displaced by wash liquor. The ideal displacement ratio would be 1.0 where ideal wash flow existed; however, the ideal situation is not obtained. The displacement ratio is primarily a function of the dilution factor but is influenced by such factors as air entrainment in the pulp, pulp uniformity, and the temperature of the system. The displacement ratio can be determined by an analysis of the amount of dissolved solids which remain in the pulp after washing compared to the dissolved solids which would be in the pulp at the same consistency without any wash flow.
Wash Liquor Ratio (WLR): The ratio between applied wash liquor and the liquor discharged from the washers with pulp suspension per unit of pulp. In multistage countercurrent washing operations, the WLR for each washing operation stage is expressed as the ratio of volume of wash liquor applied to the pulp to the volume of liquor contained in the pulp which has been treated in the individual washing stage.
Two control methods have been generally employed by the industry to control pulp washing systems in the past. These methods are directed to controlling and optimizing the flow of wash water into the system. In the first method the pulp flow rate for the entire set of washers is estimated to be constant and the pulp flow rate is calculated for the first washer. The density of the liquor displaced from the first washing stage is measured periodically and the shower water flow on the last stage washer is then set by the operator based on this measurement. The system can be out of balance several times in the interval between measurements without detection by the operator. The average liquor solids content can be on target yet the system can be inefficient in producing both high washing losses and excessive water to be evaporated because an overwash for part of the time cannot make up for an insufficient wash the other part of the time.
Moreover, any action taken by the operator would occur long after the change in washing conditions because of the bulk inertia of the large volume of liquor in the filtrate tank and time delay before the last stage shower water reaches the first stage. Consequently, operator shower-control actions in actual situations are based on trial and error and can be grossly out of sync with changes in washing conditions. The net effect is that excessive amounts of shower water are used and soda loss fluctuates widely with a generally high average.
In the second control method, the conductivity of the liquid displaced from the pulp mat in the last washing step is measured and this measurement is used to adjust the amount of fresh washing liquid in the last washing stage.
This system is an improvement over the first stage liquor density target control method described above but has several disadvantages. The large bulk inertia of the filtrate tank, which has a buffering effect, makes the filtrate conductivity determined after the tank insensitive to the soda loss variations. Another disadvantage of this method is a poor correlation between the measured soda content of filtrate and soda content of the liquor discharged with pulp from the washer.
One proposed system employs feedforward control based on an estimate of dissolved solids in the pulp slurry entering the washing system. Flow rates and conductivity measurements made on-line as the pulp slurry enters the washing zone are used to estimate the amount of dissolved solids using mass balances of liquors and of dissolved solids. The estimate of the flow of dissolved solids leaving the next to last washing stage is used to adjust washing variables in the last stage suitably. The slurry leaving the next to last washer is high consistency pulp, 10-15%, which must be diluted with recycled liquor to 1.2-1.5% consistency before it enters the last washer. The estimate of dissolved solids entering the last washer is determined from the difference between the measurements on the diluted pulp slurry and on the diluting liquor. This approach avoids the problem of making measurements on high consistency pulp, but because of the large dilution, small errors in these measurements generate large errors in the estimate of dissolved solids entering the washer.
Another proposal incorporates on-line determination of dilution factor and uses feedback control to maintain a constant dilution factor by changing the shower flow to the washers. Capacitive or microwave moisture sensors are used to determine the amount of water content present in a pulp web just before removal of the pulp in the form of a mat from the washing system. The disadvantage of this method lies in the difficulty of accurately measuring the water content in the mat. The conductivity variation of the liquid in the mat (amount or composition of dissolved solids) and entrapped air cause major difficulties in measuring the amount of water in the mat. A relatively small error in measuring the amount of water in the mat can cause a substantial inaccuracy in estimating the dilution factor which is determined as the difference of two flows, shower water and water discharged with pulp.
Another proposed system of pulp washing control is based on an on-line determination of the soda loss with pulp leaving the last stage of the washers. Pulp leaving the last stage of washers is diluted to 2-5% weight consistency, then the consistency and flow of diluted pulp and the flow of diluent is measured. Based on these measurements, the amount of pulp and liquid leaving the washers is determined. The concentration of dissolved solids in the diluted pulp slurry is determined calorimetrically. In order to compensate for dissolved solids, which are introduced into pulp suspension with diluent, the concentration of dissolved solids in the diluent must also be determined calorimetrically.
Although this method might be used for feedback control of the washing process based on on-line determination of soda losses, its applicability is limited because it requires an uneconomical dilution of the washed pulp going into storage. The large number of measurement instruments required limits the accuracy of the system because of cumulative measurement errors.
Another problem with pulp washing control is to optimize the performance of the washers themselves. Information about soda loss values alone is insufficient to perform washer optimization, because an increase in soda loss might be caused by a change in a variety of factors, for example, insufficient shower water applied (increase of pulp production rate), increased soda load (more soda per ton of pulp comes from digester in the black liquor and pulp slurry) or variation of washing conditions at the washers (high ingoing pulp slurry consistency, low level of pulp slurry in the washer vat, etc.). None of the methods discussed above can differentiate the actual cause of the increased soda loss or apply the appropriate control actions in order to compensate for the change and return the pulp washing system to optimum performance. Generally, the methods proposed to date assume that the washers have been set to optimal performance by the operator. That assumption may be valid for steady state pulp processing conditions, but it is not valid for dynamic pulp processing conditions under which washer efficiency and thus overall system efficiency may decrease significantly.